1. Field of the Invention
The present invention relates to a mounting structure for, and a method of, mounting a piezoelectric element to a circuit board.
2. Description of the Prior Art
Extended use is made of some electronic parts such as piezoelectric filters which utilize piezoelectric effect. Such piezoelectric filters include surface elastic wave filters which utilize propagation of surface elastic waves.
Referring to FIG. 1, there is illustrated a chip type surface wave filter as an example of a piezoelectric element. The piezoelectric element 5 includes connecting electrodes 9 formed in the vicinity of opposite ends of the piezoelectric element and a vibration region E located between the connecting electrodes 9. The piezoelectric element 5 uses a glass plate 15 as an element substrate on which comb electrodes are provided. A zinc oxide (ZnO) film 18 is formed so as to cover the comb electrodes for formation of the vibration region E of the piezoelectric element 5.
The piezoelectric element 5 has its length L greater than its width W, and has a ratio L/W ranging from 3 to 5 to exhibit its elongated structure.
In the prior art, to mount the piezoelectric element 5 on a circuit board 10 the piezoelectric element 5 is directly connected with and fixed to the circuit board 10 through a solder 7. This produces a problem in that application of mechanical or thermal stress to the circuit board 10 causes the stresses to be directly exerted to the piezoelectric element 5 through connection portions. The piezoelectric element 5, which has the elongated structure, is therefore likely to be bent in the direction of the length thereof (i.e., an arrow B in FIG. 1) as the stress is exerted thereon. The piezoelectric element 5, being formed with the glass plate 15, is hard and fragile and hence is likely to be cracked in the vicinity of the solder connection portions if the piezoelectric element is made to bend as described above. There is also a problem of the solder 7 being exfoliated to result in the electric characteristics of the piezoelectric element 5 being deteriorated.
Further, another problem occurs upon mounting the piezoelectric element 5 on a resin substrate and the like. The thermal expansion coefficients of the glass and the resin are different from each other (thermal expansion coefficient of the glass is approximately 3-5.times.10.sup.-6 while that of the epoxy resin being approximately 15.times.10.sup.-6) and hence any stress due to the difference between the thermal expansion coefficients may cause the piezoelectric element 5 to crack.